Metal spring contacts are used for electrically connecting integrated circuit chips or dies to circuit boards or other devices and may also be used as probe needles on a probe card. Spring contacts allow for reduced pitch, and thus, for smaller devices.
Spring contacts may be formed by depositing a release layer of material and then depositing at least two layers of stress engineered spring metal. The spring metal may be a molybdenum-chrome alloy or a nickel-zirconium alloy, as examples. The spring metal is patterned to form spring contacts and the release layer is patterned to release a free end of the spring contact. In reaction to the stresses engineered into the spring metal, the free end of the spring contact curls up. To increase the conductive and spring qualities of the spring contact, the contact may then be cladded or overplated with another material.
Each layer of spring metal has a stress introduced into it. The stress introduction may be accomplished in variety of ways during a sputter depositing of the spring metal, including adding a reactive gas to the plasma, depositing the metal at an angle and changing the pressure of the plasma gas. A compressive or a tensile stress is introduced into each layer.
Spring metals are typically brittle, particularly those that retain large stresses such as those used to make spring contacts. According to Griffith crack theory, under compression, brittle materials are strong, but under tension, cracks readily develop and propagate. For spring contacts, during spring release, if the materials are too brittle, the springs will break off in solution, leaving behind micro-machined shrapnel in the release etch. This is particularly problematic when surface flaws are present. Film brittleness has been seen to a greater degree as the spring formation process is scaled up to mass production.
The engineered stress through the thickness of two layers of deposited spring metal is shown in FIG. 1. Here the spring has a total thickness of 1 micron and a +/−1 Giga Pascal (GPa) stress variation. Stated another way, this is a 1 micron spring with a stress variation (Δσ) of 2 GPa. The stress in the layers prior to spring release is indicated by the thick solid line, and the stress profile through the thickness is indicated by the thin solid line. A dashed vertical line indicates the position within the film thickness of the neutral axes, i.e. the point inside the spring that has no change in strain before and after release.
After release, the bottom surface of the spring is placed under tension while the top surface is placed under compression. The tensile loading of the bottom surface may promote crack propagation.